Article pubs.acs.org/JPCC
Methylation of Ethene by Surface Methoxides: A Periodic PBE+D Study across Zeolites M. N. Mazar,‡ S. Al-Hashimi,§ A. Bhan,*,‡ and M. Cococcioni†,‡ ‡
Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States § Department of Chemical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates S Supporting Information *
ABSTRACT: The role of zeolite topology in the stepwise methylation of ethene by surface methoxides was investigated. Density functional theory was employed in the determination of reaction mechanisms and energy barriers. Elementary steps were studied across multiple frameworks (i.e., BEA, CHA, FER, MFI, and MOR) constituting a wide variety of confinement environments. Surface methoxides were found to react with ethene through a transition state containing planar CH3 species, which was best stabilized at the intersection of the 10-membered ring channels of MFI. A cyclopropane reaction intermediate was found in all cases; its decomposition necessitated a transition state containing a primary carbocation, which was best stabilized within the 8-membered ring side pockets of MOR. The activation energies corresponding to each transition state geometry depend upon different aspects of the local pore topology, implying that confinement effects can not be simply correlated to pore size.
1. INTRODUCTION Methanol is reliably produced through gasification and the subsequent catalytic conversion1,2 of synthesis gas from most carbon sources, including natural gas,3 coal,4,5 waste fractions of crude oil, tar sands, shale oil, and biomass.6,7 The conversion of methanol to olefins (MTO) or hydrocarbons (MTH), by reaction over microporous proton-form zeolite catalysts, represents the final step in the production of commodity chemical precursors8−15 and liquid fuels16−30 from these alternative carbon sources. However, detailed investigations of the elementary steps of the MTH process, and the influence of zeolite topology, are necessary in order to understand the wide variation in product selectivities that has been observed between zeolite frameworks.16,31−33 The formation of the first chemical bond between carbon atoms was initially postulated to occur through the direct coupling of surface-bound C1 species (e.g., physisorbed methanol or methoxides).23 However, through the rigorous elimination of hydrocarbon impurities in the methanol feed and catalysts, zeolite [H-ZSM-5 (MFI framework)] and zeotype [H-SAPO34 (CHA framework)] materials, in situ NMR analysis showed a dependence of the kinetic induction period on the presence of chance impurities.25,26 The reaction of methanol over proton-form zeolites results in the equilibration of methanol, DME, surfacebound methoxides (CH*3 ), Brønsted acid sites, and water.34,35 Direct mechanisms (e.g., involving methylcarbenes36 or trimethyloxonium ylides) necessarily require the activation of C−H bonds; however, pulsing a 1:1 ratio of d0:d6 DME at 250 °C over H/D-SAPO-34 revealed a binomial distribution among © 2012 American Chemical Society
d0, d3, and d6 DME species, proving that C−O bonds are activated while C−H bonds remain intact.35 Further, theoretical studies,37−39 conducted at the B3LYP/6-31g(d) level of theory on pentatetrahedral (5T) clusters, have found that all direct coupling mechanisms have at least one elementary step with an intrinsic activation energy greater than 200 kJ mol−1. These findings indicate that methanol conversion proceeds despite negligible contributions from the direct coupling of sorbed C1 species. Research efforts have increasingly focused on an indirect mechanism,19,40 based on successive methylation steps. In this process, the repeated methylation of light olefins13 results in higher alkenes; arenes and alkanes are formed from aromatization and H-transfer steps, respectively. The successive methylation of these arenes produces higher polymethylbenzene species. Both alkenes41−43 and arenes44,45 have been reported as “cocatalysts” in the conversion of methanol. Theoretical and isotopic labeling studies8,15,20,21,24,26,28,34,46−56 have identified alkenes and arenes as, so-called, “hydrocarbon pool” species, which participate in dual “co-catalytic” cycles and serve as scaffolds for methylation and cracking steps. In the arene cycle, a structure−function relationship between zeolite topology and entrained species has been found through in situ 13C magic angle spinning (MAS) nuclear magnetic resonance (NMR)49 and digestion of used zeolite catalysts by hydrogen fluoride.49,57 Received: June 18, 2012 Revised: August 16, 2012 Published: August 17, 2012 19385
dx.doi.org/10.1021/jp306003e | J. Phys. Chem. C 2012, 116, 19385−19395
The Journal of Physical Chemistry C
Article
form a surface propoxide, and (iii) the desorption of the surface propoxide intermediate (through β-H elimination65,76) to form propene and a Brønsted acid site. Olefin methylation steps were isolated by Hill et al.56 over H-MFI, H-MOR, H-BEA, and H-FER by measuring reaction kinetics at low olefin conversions (
